1. Field of the Invention
The present invention relates to an interface between a network for transmitting a bandwidth-compressed signal and a network for transmitting non-bandwidth-compressed signal. Particularly, in an interface between a switching node of a radio access network for transmitting a bandwidth-compressed audio signal and a switching node of a core network for transmitting a non-bandwidth-compressed audio signal opposing to the radio access network in a communication network to which the radio access network and the core network are connected, the present invention relates to an insertion/removal control of a transcoder provided in the switching node of the core network for decoding the bandwidth-compressed audio signal transmitted in the radio access network. The switching node of the radio access network is referred to as RNC (Radio Network Controller). As the switching node of the core network having a transcoder, a MSC (Mobile Switching Center) having a switching function and a MGW (Media Gateway Server) having a gateway switching function are known. Although the MSC and MGS are different from each other in view of their functions in the core network, the MSC and MGS operate in the same way in view of the insertion/removal control of the transcoder. Therefore, in the following description, the MSC will be described as a typical example of the switching node of the core network, which includes the transcoder.
2. Description of the Related Art
As a new mobile communication system, a system, which is constructed with a core network constructed as a fixed network for performing a position control, a call control and a service control and a radio access network for terminating a radio technology and having an interface between the switching node RNC of the radio access network and the switching node MSC of the core network defined as Iu interface, has been proposed. The Iu interface is defined as a hierarchic structure including a control plane for exchanging control signals between switching nodes and a user plane for exchanging transmission signals and control signals between users. The user plane is constructed with a transmission portion for transmitting a transmission signal such as an audio signal and/or data as AAL2 cell in ATM and a control signal portion for performing a negotiation between users. When a communication is performed between a calling terminal and a called terminal, the audio signal and/or the data signal is transmitted as a packet signal in ATM after a negotiation between the terminals is performed through the user plane.
The audio signal is transmitted as a 64 kbps PCM signal within the core network, since, in the core network, there are communications with a fixed cable telephone network and communications with other radio access networks. This is because the audio signal is standardized such that it is transmitted as the 64 kbps PCM signal by taking the cable telephone network and the interface with respect to other networks into consideration. On the other hand, in order to effectively utilize radio frequency, which is finite resource, the audio signal to be transmitted is bandwidth-compressed. A codec for bandwidth-compressing the audio signal is provided in each terminal so that the audio signal is transmitted in the radio access network as a narrow band signal of, for example, 9.6 kbps. Therefore, a transcoder (TC) for converting a bandwidth-compressed audio signal into a 64 kbps PCM signal, transmitting the latter to the side of the core network or converting a 64 kbps PCM signal into a bandwidth-compressed audio signal and transmitting the latter to the radio access network is provided in the switching node MSC of the core network to perform the conversion between the bandwidth-compressed audio signal and the 64 kbps PCM signal. FIG. 9 shows a construction for converting the bandwidth-compressed audio signal into the 64 kbps PCM signal and transmitting the latter signal.
In FIG. 9, DHT (Diversity Handover Trunk) is provided in the switching node RNC on each of a calling side and a called side to play a hand-over function in a state of diversity.
Assuming that the called side is a radio terminal utilizing the same audio compression coding system, the audio signal is coded/decoded by at least four coding/decoding devices in the passage from the calling side to the called side, that is, a codec of the calling terminal, a transcoder of the switching node MSC on the calling side, a transcoder of the switching node of the called terminal and a codec of the called terminal. Therefore, distortion of the transmission signal due to the coding/decoding method is accumulated, resulting in a degraded audio signal quality. If transcoders are inserting into all channels of the switching node MSC, the cost therefor becomes high and a signal delay may occur due to the signal conversions.
In a case where one audio compression coding system is used in the mobile communication network, the calling and called terminals have identical codecs. Therefore, there is no need of converting the bandwidth-compressed signal into the 64 kbps PCM signal by using the transcoder of the switching node MSC. That is, there is no problem if the audio signal is transmitted as a bandwidth compression coded, narrow band signal within the core network without using the transcoder. In such case, it is possible to reduce the installation cost of transcoders and, further, the quality of the audio signal can be improved since the number of coding/decoding steps to be performed in the communication is reduced. Therefore, in the mobile communication network using one and the same compression coding system, a system has been employed in which a bandwidth-compressed audio signal is transmitted as it is by bypassing the transcoder in the switching node of the core network. This system is referred to as Transcoder Free Operation (TrFO).
FIG. 10 shows a connection for the Transcoder Free Operation.
In the above mentioned new mobile communication system, it has been proposed to use an AMR (Adaptive Multi Rate Codec) as the compression coding system. The AMR can determine its conversion rate dynamically in a range from 4.75 kbps to 12.2 kbps, for example. Further, the class of AMR is assigned dependent upon tolerable error rate and the assignment of the coding frame of the AMR is performed by transmitting a RFCI (Rab sub-Flow Combination Indicator) information assigning a SDU (Service Data Unit) size of a table describing the conversion type between users by using the user plane and negotiating mutually between the calling terminal and the called terminal.
As mentioned, in the Transcoder Free Operation using the AMR compression coding system, the RFCI information for compression-coding is determined by users in the user plane of the interface between the switching node MSC of the core network and the switching node RNC of the radio access network. Therefore, when the transcoder is inserted in a communication path, the switching node MSC of the core network must insert the transcoder by executing initializing procedures between the switching node MSC and the switching node RNC of the radio access network.
It is assumed that a communication between a calling terminal and a called terminal is performed by bypassing a transcoder in a switching node MSC using the TrFO connection. In such case, one of the terminal, which is moving, enters into an area covered by a switching node RNC of a different radio access network while switching a base station by the hand-over. This state is shown in FIG. 9. In this case, there may be a case where the terminal moves to a switching node RNC of a different communication carrier. For example, in Europe, there may be a communication crossing national boundaries. In such case, when relocation is performed to a switching node RNC of a different carrier, it may be impossible to continue the TrFO connection since the same RFCI information is not always used.
In such case, it is necessary to initialize a local side switching node MSC, that is, the relocating terminal side, of a core network by inserting a transcoder, convert an audio signal into a standard 64 kbps PCM signal, further convert the latter into a bandwidth-compressed signal corresponding to a remote terminal side and transmit the bandwidth-compressed signal to the remote terminal side. This is because there is no guarantee that the same RFCI information for the bandwidth-compression as that previously used can be used for a new switching node RNC.
As described, when a terminal moves to a switching node RNC of a new radio access network under the TrFO connection, a transcoder is inserted by performing an initialization of transcoder with respect to the switching node of the new radio access network.
Therefore, when the terminal enters into the area covered by the new switching node RNC and the transcoder in the switching node MSC is inserted, it is necessary to insert at least four coding/decoding devices as mentioned previously and degradation of tone quality can not be avoided.
In order to solve this problem, a method has been proposed (Through Connection and Iu User Plane Initialization during TrFO establishment 2000.7.16 Siemens 3GPP TSG CN WG4#03 TdocN4-00476). In the proposed method, a TrFO connection between a switching node RNC of a radio access network on a remote side in which a terminal is not moved and a switching node MSC of a core network is kept continued and a negotiation is performed between the switching node MSC of the core network and the switching node RNC of the new radio access network on the side of the moving terminal by inserting a transcoder to inquire of the new switching node about RFCI information. When the RFCI information is the same as that used in the previous communication and satisfies the TrFO condition, the system is controlled such that it is shifted to the TrFO.
In this proposal, however, the procedure for shifting the TrFO by inserting/removing the transcoder is complicated, compared with the system in which the negotiation is performed after the re-initialization is performed once. That is, when a negotiation is performed between the new switching node RNC of the radio access network and the switching node MSC of the core network and the RFCI information is different from that used for the previous TrFO connection, a re-initialization including the remote side switching node RNC becomes necessary, so that the procedure therefor becomes complicated.